1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus of a stand-alone type, a network type or the like for forming monochrome images and/or color images such as a copying machine, a printer, a facsimile machine or a composite machine including a combination of two or more of them.
2. Description of Related Art
The above kind of image forming apparatus in the prior art includes an electrostatic latent image carrier, a charging device and a developing device, and further includes a charging power source or supply for charging a surface of the electrostatic latent image carrier to a predetermined potential and a developing bias power source for applying a developing bias voltage to the developing device for developing an electrostatic latent image.
In this kind of image forming apparatus, the charging device to which a charging voltage is applied from the charging power source charges the surface of the electrostatic latent image carrier, and the electrostatic latent image is formed on a region thus charged, and is developed by the developing device to which the developing bias voltage is applied from the developing bias power source.
In the image forming apparatus such as a printer, it is now demanded to reduce the number of parts for providing a compact structure of the apparatus.
For satisfying this demand, the charging power source and the developing bias power source may be formed of a common power source.
However, the common power source serving as both the charging power source and the developing bias power source applies a charging voltage of a value for practical use in image formation to the charging device, and also applies the developing bias voltage of a value for practical use in image formation to the developing device when the voltages applied to the charging device and the developing device are raised.
As a result, developer moves onto the electrostatic latent image carrier when the voltages applied to the charging device and the developing device are raised so that the developer is wasted. The developer is likewise wasted when the voltages applied to the charging and developing devices are lowered.
For performing, e.g., reversal development of the electrostatic latent image with two-component developer containing carrier and negatively chargeable toner, the common power source applies the negative charging voltage of the value for practical use in the image formation to the charging device, and also applies the negative developing bias voltage of the value for practical use in the image formation when the voltages applied to the charging and developing devices are raised. Thereby, a surface region of the electrostatic latent image carrier, which is not yet charged by the charging device, reaches the developing device supplied with the developing bias voltage before a surface region of the electrostatic latent image carrier, which is charged by the charging device, reaches the developing device. Therefore, the negatively charged toner moves from the developing device onto this uncharged region. This wastes the toner.
When the voltages applied to the charging and developing devices are to be lowered, the common power source simultaneously stops the supply of voltages to the charging device and the developing device so that a surface region of the electrostatic latent image carrier, which is already negatively charged by the charging device, reaches the developing device, to which the voltage is not applied, and the positively charged carrier move onto this charged region from the developing device. This wastes the carrier.
For example, in an image forming apparatus, which perform reversal development of an electrostatic latent image with a one-component developer primarily made of negatively chargeable toner, negatively charged toner moves onto a region of the electrostatic latent image carrier, which is not yet charged, when the voltages applied to the charging and developing devices are raised so that the toner is wasted.